The disclosures of Japanese Patent Applications Nos. 2000-374482 filed on Dec. 8, 2000, 2000-388978 filed on Dec. 21, 2000 and 2001-9306 filed on Jan. 17, 2001, each including the specification, drawings and abstract, are incorporated herein by reference in their entireties.
1. Field of the Invention
The invention relates to an emission control apparatus of an internal combustion engine.
2. Description of the Related Art
In a known internal combustion engine, a NOx occluding member that occludes NOx when the air-fuel ratio of an inflow exhaust gas is on a fuel-lean side of a stoichiometric fuel-air ratio and that releases occluded NOx and reduces NOx by a reducing agent contained in exhaust gas when the inflow exhaust gas air-fuel ratio changes to the fuel-rich side of the stoichiometric fuel-air ratio is disposed within an engine exhaust passage. During a combustion mode under a fuel-lean air-fuel ratio condition, NOx in exhaust gas is occluded into the NOx occluding member. When NOx is to be released from the NOx occluding member, the air-fuel ratio of exhaust gas that flows into the NOx occluding member is changed toward the rich side.
In order to change the air-fuel ratio of exhaust gas flowing into the NOx occluding member from the fuel-rich side to the fuel-lean side when the release of NOx from the NOx occluding member is completed in an internal combustion engine as described above, there has been proposed an internal combustion engine (Japanese Patent Application Laid-Open No. 2000-104533) in which a NOx sensor capable of detecting the concentration of NOx in exhaust gas is disposed in an engine exhaust passage downstream of the NOx occluding member, and in which when the NOx concentration detected by the NOx sensor decreases to or below a predetermined concentration, the release of NOx from the NOx occluding member is considered to have been completed, and the air-fuel ratio of exhaust gas flowing into the NOx occluding member is changed from the rich side to the lean side.
However, while NOx is being released from the NOx occluding member, the released NOx is reduced by the reducing agent, and therefore is not released in the form of NOx. Therefore, during the release of NOx from the NOx occluding member, the NOx concentration detected by the NOx sensor remains substantially at zero. Therefore, it is not possible to determine whether the release of NOx from the NOx occluding member has been completed, through the use of the NOx sensor.
If the air-fuel ratio of exhaust gas flowing into the NOx occluding member is shifted to the rich side in the aforementioned internal combustion engine, the air-fuel ratio of exhaust gas flowing out of the NOx occluding member is normally a slightly lean air-fuel ratio during the NOx releasing operation of the NOx occluding member. After the release of NOx from the NOx occluding member is completed, the air-fuel ratio of exhaust gas flowing out of the NOx occluding member shifts to the rich side.
In order to change the air-fuel ratio of exhaust gas flowing into the NOx occluding member at the time of completion of the release of NOx from the NOx occluding member in an internal combustion engine as described above, there has been proposed an internal combustion engine (see Japanese Patent Application Laid-Open No. 8-232646) in which an air-fuel ratio sensor that produces an output whose level is proportional to the air-fuel ratio of exhaust gas is disposed in an exhaust passage downstream of a NOx occluding member, and in which after the air-fuel ratio of exhaust gas flowing into the NOx occluding member is changed from the lean side to the rich side so as to release NOx from the NOx occluding member, it is determined that the release of NOx from the NOx occluding member is completed when the rate of change in the output level of the air-fuel ratio sensor when the air-fuel ratio of exhaust gas flowing out of the NOx occluding member changes from the lean side to the rich side exceeds a predetermined rate of change.
The output level of the air-fuel ratio sensor changes in a good response to completion of the release of NOx from the NOx occluding member. Therefore, by determining whether the NOx releasing operation is completed based on a change in the output level of the air-fuel ratio sensor as mentioned above, it becomes possible to change the air-fuel ratio of exhaust gas flowing into the NOx occluding member from the rich side to the lean side in a good response to completion of the NOx releasing operation. However, at the time of completion of the release of NOx, the output level of the air-fuel ratio sensor changes in various fashions, depending on performance variations among air-fuel ratio sensors and NOx occluding members, or time-depending changes thereof. Therefore, the rate of change in the output level exceeding the predetermined rate of change does not necessarily mean that the NOx releasing operation has been completed. Therefore, there is a drawback in the conventional art. That is, it is difficult to change the air-fuel ratio from the fuel-rich side to the fuel-lean side at the time of completion of the release of NOx.
Through experiments and researches on NOx occluding members carried out by the present inventors and the like, it has been found that if an NOx occluding member is supplied with a reducing agent in an amount that is greater than the amount needed to reduce the amount of NOx occluded in the NOx occluding member when the air-fuel ratio flowing into the NOx occluding member is changed to the fuel-rich side, that is, if the air-fuel ratio of exhaust gas flowing into the NOx occluding member continues to be on the rich side even after completion of the release of NOx from the NOx occluding member, a surplus amount of reducing agent that has not been used to release NOx from the NOx occluding member and reduce NOx is discharged from the NOx occluding member in the form of ammonia.
Therefore, if the amount of ammonia discharged from the NOx occluding member is determined, the surplus amount of the reducing agent is determined, which in turn makes it possible to determine the amount of the reducing agent needed to reduce the amount of NOx occluded in the NOx occluding member. If the amount of the reducing agent needed to reduce the NOx occluded in the NOx occluding member is determined, it become possible to change the air-fuel ratio of exhaust gas flowing into the NOx occluding member at the time of completion of the release of NOx from the NOx occluding member by setting a degree of fuel-richness and a duration of rich-side shift of the air-fuel ratio of exhaust gas flowing into the NOx occluding member so as to supply the needed amount of the reducing agent. Furthermore, if the amount of the reducing agent needed to reduce the NOx is determined, the amount of NOx occludable by the NOx occluding member can be determined, which in turn makes it possible to determine the degree of deterioration of the NOx occluding member.
Thus, given a surplus amount of the reducing agent is determined, the state of the NOx occluding member can be recognized, and the release of NOx from the NOx occluding member can be appropriately controlled.
Furthermore, if the discharge of ammonia from the NOx occluding member is monitored when the air-fuel ratio of exhaust gas flowing into the NOx occluding member is shifted to the rich side so as to release NOx from the NOx occluding member, it is possible to determine whether the release of NOx from the NOx occluding member has been completed.
It is an object of the invention to provide an emission control apparatus of an internal combustion engine capable of appropriately controlling the release of NOx from a NOx occluding member.
A first aspect of the invention is an emission control apparatus of an internal combustion engine in which a NOx occluding member that occludes a NOx when an air-fuel ratio of an inflow exhaust gas is on a fuel-lean side, and that, when the air-fuel ratio of the inflow exhaust gas changes to a fuel-rich side, allows the NOx occluded to be released and reduced by a reducing agent contained in the exhaust gas is disposed in an exhaust passage of the engine, and in which the NOx in the exhaust gas is occluded into the NOx occluding member when a combustion is conducted under a fuel-lean air-fuel ratio condition, and when the NOx is to be released from the NOx occluding member, the air-fuel ratio of the exhaust gas flowing into the NOx occluding member changed to the fuel-rich side. In this aspect, when the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side, a surplus amount of a reducing agent that is not used to release and reduce the NOx occluded in the NOx occluding member is let out in a form of ammonia from the NOx occluding member. Furthermore, a sensor capable of detecting an ammonia concentration is disposed in the exhaust passage downstream of the NOx occluding member. A representative value that indicates the surplus amount of the reducing agent is determined from a change in the ammonia concentration detected by the sensor.
In the first aspect, the representative value may be an integrated value of the ammonia concentration detected by the sensor.
In the first aspect, the representative value may be a maximum value of the ammonia concentration detected by the sensor.
In the first aspect, it is possible that as the representative value increases, a total amount of the reducing agent supplied to the NOx occluding member when the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side may be reduced.
In the first aspect, it is possible that as the representative value increases, a time during which the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is kept on the fuel-rich side may be reduced.
In the first aspect, a reference value may be pre-set regarding the representative value. If the representative value becomes greater than the reference value, a total amount of the reducing agent supplied to the NOx occluding member when the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side may be reduced. If the representative value becomes less than the reference value, the total amount of the reducing agent supplied to the NOx occluding member when the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side may be increased.
In the first aspect, if the representative value becomes greater than the reference value, a time during which the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is kept on the fuel-rich side maybe reduced. If the representative value becomes less than the reference value, the time during which the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is kept on the fuel-rich side may be increased.
In the first aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-lean side to the fuel-rich side if a predetermined set value is exceeded by the NOx concentration detected by the sensor while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the first aspect, the emission control apparatus may further include amount-of-occluded-NOx estimating device that estimates an amount of the NOx occluded in the NOx occluding member. A fuel-rich time interval for temporarily changing the air-fuel ratio of the exhaust gas flowing into the NOx occluding member to the fuel-rich side may be controlled based on the amount of the NOx estimated by the amount-of-occluded-NOx estimating device.
In the first aspect, the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be temporarily changed from the fuel-lean side to the fuel-rich side when the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device exceeds an allowable value.
In the first aspect, the emission control apparatus may further include NOx occluding capability estimating device that estimates a NOx occluding capability of the NOx occluding member. The allowable value may be reduced as the NOx occluding capability estimated by the NOx occluding capability estimating device decreases.
In the first aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas. The air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-lean side to the fuel-rich side if the NOx concentration detected by the sensor exceeds a predetermined set value although the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device remains less than or equal to the allowable value while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the first aspect, the sensor maybe capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas. The allowable value may be reduced if the NOx concentration detected by the sensor exceeds a predetermined set value although the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device remains less than or equal to the allowable value while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the first aspect, a degree of deterioration of the NOx occluding member may be detected based on the representative value.
In the first aspect, it maybe determined that the degree of deterioration of the NOx occluding member increases with a decrease in an amount obtained by subtracting the surplus amount of the reducing agent from a total amount of the reducing agent supplied to the NOx occluding member.
In the first aspect, when the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side, a degree of fuel-richness may be reduced with an increase in the degree of deterioration of the NOx occluding member.
A second aspect of the invention is an emission control apparatus of an internal combustion engine in which a NOx occluding member that occludes a NOx when an air-fuel ratio of an inflow exhaust gas is on a fuel-lean side and that releases the occluded NOx when the air-fuel ratio of the inflow exhaust gas changes to a fuel-rich side is disposed in an exhaust passage of the internal combustion engine, and in which the NOx in the exhaust gas is occluded into the NOx occluding member when a combustion is conducted under a fuel-lean air-fuel ratio condition, and the air-fuel ratio of the exhaust gas flowing into the NOx occluding member to the fuel-rich side is changed when the NOx is to be released from the NOx occluding member. In this aspect, a sensor capable of detecting an ammonia concentration is disposed in the exhaust passage downstream of the NOx occluding member. It is determined that a release of the NOx from the NOx occluding member is completed, if the ammonia concentration detected by the sensor starts to rise while the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is kept on the fuel-rich side so as to release the NOx from the NOx occluding member.
In the second aspect, the sensor may generate an output signal having a level proportional to the ammonia concentration, and it may be determined that the release of the NOx from the NOx occluding member is completed, if the level of the output signal of the sensor exceeds a predetermined set value while the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is kept on the fuel-rich side so as to release the NOx from the NOx occluding member.
In the second aspect, the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-rich side to the fuel-lean side if it is determined that the release of the NOx from the NOx concentration is completed.
In the second aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-lean side to the fuel-rich side if a predetermined set value is exceeded by the NOx concentration detected by the sensor while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the second aspect, the emission control apparatus may further include amount-of-occluded-NOx estimating device that estimates an amount of the NOx occluded in the NOx occluding member. A fuel-rich time interval for temporarily changing the air-fuel ratio of the exhaust gas flowing into the NOx occluding member to the fuel-rich side may be changed based on the amount of the NOx estimated by the amount-of-occluded-NOx estimating device.
In the aforementioned aspect, the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be temporarily changed from the fuel-lean side to the fuel-rich side when the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device exceeds an allowable value.
In the aforementioned aspect, the emission control apparatus may further include NOx occluding capability estimating device that estimates a NOx occluding capability of the NOx occluding member. The allowable value may be reduced as the NOx occluding capability estimated by the NOx occluding capability estimating device decreases.
In the aforementioned aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-lean side to the fuel-rich side if the NOx concentration detected by the sensor exceeds a predetermined set value although the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device remains less than or equal to the allowable value while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the aforementioned aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas, and the allowable value maybe reduced if the NOx concentration detected by the sensor exceeds a predetermined set value although the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device remains less than or equal to the allowable value while the combustion is conducted under the fuel-lean air-fuel ratio condition.
A third aspect of the invention is an emission control apparatus of an internal combustion engine in which a NOx occluding member that occludes a NOx when an air-fuel ratio of an inflow exhaust gas is on a fuel-lean side, and that, when the air-fuel ratio of the inflow exhaust gas changes to a fuel-rich side, allows the NOx occluded to be released and reduced by a reducing agent contained in the exhaust gas is disposed in an exhaust passage of the engine, and in which air-fuel ratio detector is disposed in the exhaust passage of the engine downstream of the NOx occluding member. In the emission control apparatus, the NOx in the exhaust gas is occluded into the NOx occluding member when a combustion is conducted under a fuel-lean air-fuel ratio condition. The air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side when the NOx is to be released from the NOx occluding member. At a time near completion of the release the NOx from the NOx occluding member, the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed from the fuel-rich side to the fuel-lean side if an output signal level of the air-fuel ratio detector exceeds a reference level while the output signal level of the air-fuel ratio detector is changing toward a level that indicates a fuel-rich air-fuel ratio. In this aspect, when the air-fuel ratio of the exhaust gas flowing into the NOx occluding member is changed to the fuel-rich side, a surplus amount of a reducing agent that is not used to release and reduce the NOx occluded in the NOx occluding member is let out in a form of ammonia from the NOx occluding member. A sensor capable of detecting an ammonia concentration is disposed in the exhaust passage downstream of the NOx occluding member. The reference level is changed so that the air-fuel ratio of the exhaust gas is changed from the fuel-rich side to the fuel-lean side when a release of the NOx from the NOx occluding member is completed based on a change in the ammonia concentration detected by the sensor.
In the third aspect, the representative value that indicates the surplus amount of the reducing agent may be determined from a change in the ammonia concentration detected by the sensor, and the reference level may be changed so that the representative value reaches a target value.
In the third aspect, the representative value may be an integrated value of the ammonia concentration detected by the sensor.
In the third aspect, the representative value may be a maximum value of the ammonia concentration detected by the sensor.
In the third aspect, the sensor maybe capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-lean side to the fuel-rich side if a predetermined set value is exceeded by the NOx concentration detected by the sensor while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the third aspect, the emission control apparatus may further include amount-of-occluded-NOx estimating device that estimates an amount of the NOx occluded in the NOx occluding member. A fuel-rich time interval for temporarily changing the air-fuel ratio of the exhaust gas flowing into the NOx occluding member to the fuel-rich side may be controlled based on the amount of the NOx estimated by the amount-of-occluded-NOx estimating device.
In the foregoing aspect, the air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be temporarily changed from the fuel-lean side to the fuel-rich side when the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device exceeds an allowable value.
In the foregoing aspect, the emission control apparatus may further include NOx occluding capability estimating device that estimates a NOx occluding capability of the NOx occluding member. The allowable value may be reduced as the NOx occluding capability estimated by the NOx occluding capability estimating device decreases.
In the foregoing aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas. The air-fuel ratio of the exhaust gas flowing into the NOx occluding member may be changed from the fuel-lean side to the fuel-rich side if the NOx concentration detected by the sensor exceeds a predetermined set value although the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device remains less than or equal to the allowable value while the combustion is conducted under the fuel-lean air-fuel ratio condition.
In the foregoing aspect, the sensor may be capable of detecting a NOx concentration in the exhaust gas besides the ammonia concentration in the exhaust gas. The allowable value maybe reduced if the NOx concentration detected by the sensor exceeds a predetermined set value although the amount of the NOx occluded estimated by the amount-of-occluded-NOx estimating device remains less than or equal to the allowable value while the combustion is conducted under the fuel-lean air-fuel ratio condition.